The present invention relates to process variable sensor of the type used to sense a process variable of an industrial process. More specifically, the present invention relates to characterization of pressure sensors used in process monitor and control applications.
Process transmitters are used to monitor and control industrial processes by measuring various characteristics of fluids used in the process. (As used herein, fluid includes both liquid and gas phase materials and mixtures of such materials). One process variable which is frequently measured is pressure. The pressure can be a differential pressure or a line, gage, absolute or static pressure. In some installations, the measured pressure is used directly. In other configurations, the measured pressure is used to derive other process variables. For example, the differential pressure across a restriction (primary element) in a pipe is related to fluid flow rate through the pipe. Similarly, the differential pressure between two vertical locations in a tank is related to level of liquid in the tank. Process transmitters are used to measure such process variables and transmit the measured process variable to a remote location, for example a control room. A transmission can occur over various communication mediums, for example, a two-wire process control loop.
The pressure sensors used in process transmitters provide output signals related to applied pressure. The relationship between the output signal and the applied pressure is known to vary between pressure sensors and in some applications static pressure as well. The variations are functions of the applied pressure as well as the temperature of the pressure sensor. The variations may also be functions of the static pressure. In order to improve the accuracy of measurements using such pressure sensors, each pressure sensor typically undergoes a characterization process during manufacture. The characterization process involves applying known pressures to the pressure sensor and measuring the output of the pressure sensor. This is done with multiple processors. Typically, data is also taken at different temperatures. For example, a pressure sensor might be characterized between a pressure of 0 and 250 inches. Such a characterization process might take data at 10 evenly spaced pressure intervals (25 inches, 50, inches, 75 inches, etc.). Multiple data sets can be taken at different temperatures. The data is then fit to a polynomial curve, for example using a least squares curve fitting technique. The coefficients of the polynomial are then stored in a memory of the transmitter and used to compensate subsequent pressure measurements taken by the pressure sensor.